1. Field of the Invention
The present invention relates to a method and device for measuring insulation deterioration of electric line with load, and in particular to an improved method and device for measuring and/or monitoring insulation resistance of electric line in an operating state.
2. Description of the Prior Art
Generally, a high voltage insulation resistance measuring instrument (hereinafter called megger) has been used for measuring insulation resistance of electric line in which a dc voltage is applied to the electric line, and insulation resistance is measured from a current returned to a grounding conductor. However, the insulation resistance measuring method employing the megger has the following drawbacks:
(1) Since high dc voltage is applied, a device or an element of low dielectric strength typically made of a semiconductor provided in the line as a load or the like, would be destroyed. Accordingly, the measurement should be made while disconnecting such load each time of measurement.
(2) Recently, it has become known that there is an insulation deterioration of electrical line with load, which is equivalently represented by the series coupling of resistance and capacitance in addition to one that is equivalently represented by the parallel coupling of resistance and capacitance which has been known before. However, since the megger employs a dc voltage, such resistance as represented by series coupling cannot be measured.
The various methods and devices for measuring and/or monitoring insulation resistance of electric line in operating states have been proposed but the above problem (2) has not been solved. Typical of those methods include a first method wherein a low frequency voltage is applied to electric line through electromagnetic induction via a class two grounding conductor for the transformer by such means as passing the grounding conductor through the core of the oscillation transformer of an oscillator oscillating a low frequency voltage which is a measuring signal or a transformer to which a low frequency voltage is applied, leakage current having returned to the grounding conductor is detected by a zero-phase inverter or the like, and insulation resistance is measured by detecting the effective component of the low frequency component in the leakage current; and a second method wherein the class two grounding conductor for the transformer is disconnected, an oscillator for applying a low frequency voltage being coupled in series in the disconnected part and a resistor for detecting a current having returned to the grounding conductor are connected in series, and insulation resistance is measured by detecting the effective component due to the insulation resistance of the low frequency component in the leakage current obtained across the resistor.
FIG. 1(a) and FIG. 2 show a prior art method disclosed in Japanese patent publication No. 68290/1978 (filed on Nov. 30, 1976), and FIG. 3 shows another prior art method disclosed in Japanese patent publication No. 7378/1978 (filed on July 9, 1976).
These drawings show the models in which a resistance R and a capacitance C exist in parallel between the line and the earth.
In actual situations, however, series connected R.sub.1 and C.sub.1 often exist between the line and the earth in addition to the parallel connected R and C as shown in FIG. 1(b). Hence, in conventional measuring systems based on the circuit models in which such additional elements are neglected, it is impossible to detect separately the effect of R.sub.1 and C.sub.1, thus involving great potential dangers, especially when R.sub.1 becomes small and C.sub.1 grows large, for example, when a large quantity of salt adheres to insulators.
FIG. 1(a) illustrates the above-mentioned first method. A load Z is connected across a transformer T via lines 1 and 2. Though explanation is made here about the case of the single phase two wire circuits of lines 1 and 2 to facilitate the understanding, explanation may be made similarly about the cases of single-phase three-wire circuits and three-phase three-wire circuits, etc. using the principle shown below. In FIG. 1(a), R is an insulation resistance of the electric line, and C is an earth stray capacity. An earth line (grounding conductor) EL penetrates the core of an oscillation transformer OT which generates a measuring signal of frequency f.sub.1 (different from a commercial frequency f.sub.0) or the core of a transformer to which a voltage of frequency f.sub.1 is applied. A voltage V.sub.1 [Volt] (the measuring signal is assumed to be a sine wave though it may be a rectangular wave) of frequency f.sub.1 is assumed to be induced in the earth line EL. A zero-phase inverter ZCT through which the earth line EL penetrates is for detecting leakage current. If the detected leakage current is fed to a filter FIL which permits the frequency f.sub.1 component and the leakage current of the commercial frequency component is removed, the following output ig of the filter FIL can be obtained. ##EQU1## where .omega..sub.1 =2.pi.f.sub.1.
When the output voltage of an oscillator circuit OSC is e.sub.1 [Volt], multiplication by ig with a multiplier MULT will give the following product. ##EQU2## Accordingly, when the dc component of ig.times..sqroot.2e.sub.1 sin .omega..sub.1 t is obtained by directing the output of the multiplier MULT through a low-pass filter LPF, the output OUT, i.e., the effective component becomes as follows: EQU OUT=e.sub.1 V.sub.1 /R (3)
Accordingly, if e.sub.1 and V.sub.1 are constant, the insulation resistance may be obtained by substituting the value of OUT.
As already mentioned, it is known that resistance of electric line which can be represented equivalently by the series connection of resistance R.sub.1 and capacitance C.sub.1 indicated in FIG. 1(b) in addition to that which can be represented by the parallel connection of resistance R and capacitance C indicated in FIG. 1(a) (Refer to "Illustrated Electrics" Vol. 20, No. 1, 1979, pp 23). In the first method, if the resistance represented equivalently by the resistance R.sub.1 and capacitance C.sub.1 is present, a value lower than the value measured by a megger is obtained. In the first method, since no high dc voltage is applied, insulation resistance can be measured in an operating state of electric line without disconnecting the load thus solving the aforementioned problem (1), but leaving unsolved the problem (2), i.e., the problem by the presence or absence of insulation deterioration represented equivalently in the form of series connection of resistance and capacitance and its value.
FIG. 2 illustrates the second method. In this method, a low frequency applying transformer T.sub.1 and a leakage current detecting resistor r are connected to the earth line EL in series. The second method shown in FIG. 2 is identical with the first method shown in FIG. 1 except the points in which low frequency is applied and different leakage detection method is used. The second method, however, has the same problem as the first method does.
In addition, there has been proposed a third method wherein commercial supply voltage is directly used without the application of a particular measuring low frequency voltage to electric line, as shown in FIG. 3. However, when this method is adapted to a circuit of single-phase three-wire system, leakage currents generated from two non-grounded circuits cancel each other at an earth line EL because of mutually different current direction and accurate insulation resistance cannot be measured. For this reason, the third method is used only for the single-phase two-wire system. In this case, leakage current to be returned to the earth line corresponds to a value obtained by replacing .omega..sub.1 with .omega..sub.0 (.omega..sub.0 =2.pi.f.sub.0, f.sub.0 =commercial frequency) in Eq. (1) and V.sub.1 with line voltage V.sub.0. If a voltage obtained by dividing a voltage obtained from lines 1 and 2 with a transformer T.sub.2 is e.sub.0 [Volt], the output of a multiplier MULT is equal to that to be obtained by substituting e.sub.0 v.sub.0 for e.sub.1 V of Eq. (2) and .omega..sub.0 for .omega..sub.1. Accordingly, the output of a low-pass filter LPF becomes e.sub.0 V.sub.0 /R. In the case of this method, the same problem as in the cases of the methods shown in FIGS. 1 and 2 arises. The output OUT of the low-pass filter LPF is equal to that obtained by substituting e.sub.0 V.sub.0 for e.sub.1 V.sub.1 of Eq. (5) and .omega..sub.0 for .omega..sub.1, and the output of the low-pass filter LPF at this time does not correspond to the insulation resistance measured with a megger.
Referring to FIG. 1(b), the output ig' of the filter FIL when R.sub.1 and C.sub.1 exist, can be expressed by the following equation: ##EQU3## When the calculation of ig'.times..sqroot.2e.sub.1 sin .omega..sub.1 t is made, the effective component, i.e., the output OUT of the low-pass filter LPF, at this time becomes EQU OUT'=[1/R+1/R.sub.1 {1/(1+1/.omega..sub.1.sup.2 C.sub.1.sup.2 R.sub.1.sup.2)}]e.sub.1 V.sub.1 ( 5)
From Eq. (5), when .omega..sub.1 C.sub.1 R.sub.1 &gt;1, EQU OUT'.congruent.(1/R+1/R.sub.1)e.sub.1 V.sub.1 ( 6)
when .omega..sub.1 C.sub.1 R.sub.1 .ltoreq.1, EQU OUT'.congruent.(1/R)e.sub.1 V.sub.1 ( 7)
When .omega..sub.1 C.sub.1 R.sub.1 .congruent.1, EQU OUT'.congruent.(1/R+1/2R.sub.1)e.sub.1 V.sub.1 ( 8)
From Eq. (5), insulation resistance Rg to be measured from the output OUT' becomes EQU 1/Rg=1/R+(1/R.sub.1){1/(1+1/.omega..sub.1.sup.2 C.sub.1.sup.2 R.sub.1.sup.2)} (9)
However, as can be noted from Eqs. (6), (7), and (8), the value of Rg varies according to the measuring frequency f.sub.1 =.omega..sub.1 /2.pi.. When the measuring frequency f.sub.1 is sufficiently high, 1/Rg=1/R+1/R.sub.1 from Eq. (6). On the other hand, when the measuring frequency f.sub.1 is increased to a sufficiently high level, ineffective current due to stray capacity including the third term of Eq. (4) will become notably large, calculation error of the multiplier MULT (or a synchronous detector) will become large, and a large error in the calculation of Eq. (5) may result.
For example, insulation resistance Rg to be obtained from Eq. (9) when R=500 k.OMEGA., R1=10 k.OMEGA., C=1 .mu.F, and .omega..sub.1 /2.pi.=20 [Hz] will be ##EQU4## and Rg.congruent.15.8 [k.OMEGA.] can be obtained. That is, Rg is different from the value 9.8 k.OMEGA. in which R and R.sub.1 are connected in parallel.
On the other hand, when insulation resistance viewed from the secondary side of the transformer T to the load is measured by a megger, a value according to insulation resistance R, i.e., 500 k.OMEGA. in the above example will be measured regardless of C.sub.1 and R.sub.1 since measurement is made by direct current.
When line voltage is 200 V, commercial frequency current flowing through R.sub.1 and C.sub.1 can be expressed as follows if commercial frequency .omega..sub.0 /2.pi.=50 [Hz]; ##EQU5## That is amperage is about 20 mA and power to be spent at R1 becomes 4 [W] (=10.times.10.sup.3 .times.400.times.10.sup.-6), whereby the probability of heating is very high rendering a very dangerous situation.
That is, the presence of insulation deterioration such as that represented by R.sub.1 and C.sub.1 as shown by FIG. 1(b) cannot be detected from only an insulation resistance value given by Eq. (5).
In addition, in the series connection of C.sub.1 and R.sub.1 shown in FIG. 1(b), insulation deterioration may sometimes be present at more than one point. In such case, the affected points cannot be detected either.
When deterioration of series connection of capacitance C.sub.1 and resistance R.sub.1 exists at "n" locations, that is, when capacitance Ci and resistance Ri (i=1-n) (not shown) are present in lieu of capacitance C.sub.1 and resistance R.sub.1 and as many as "n" series circuits consisting of Ci and Ri are arranged in parallel, insulation resistance Rg (n) to be measured can be expressed as follows from Eq. (5). ##EQU6##
Insulation deterioration occurring at plural points simultaneously as given in Eq. (11) is vary rare, and it is generally assumed that relatively simple insulation deterioration as given by Eq. (9) occurs in most case.